1. Field of Invention
The present invention relates to an apparatus and method for forming a thermoplastic resin foam. The apparatus comprises a screw cylinder and a screw provided drivably in the directions of plasticization and injection inside said screw cylinder. The method comprises the step of injecting an inert gas such as a carbon dioxide gas or a nitrogen gas, having at least in pressure a pressure equal to or greater than a supercritical pressure into said screw cylinder. The method also comprises the steps of permeating melted resin with the inert gas under a supercritical state inside the screw cylinder, and injecting the melted resin, permeated by the inert gas, into a mold to provide a thermoplastic resin foam.
2. Related Art
There have been suggested many apparatuses and methods for molding a thermoplastic resin foam, for example, in Japanese Unexamined Patent Publication Hei 8-258096 and No. Hei. 10-230528. The methods include the steps of melting thermoplastic resin inside the cylinder of an injection-molding machine, permeating the melted thermoplastic resin with an inert gas such as carbon dioxide gas or nitrogen gas under a supercritical state, and injecting the melted resin, permeated by the gas, into the mold. The apparatus for forming a fine foam, disclosed in the aforementioned Japanese Unexamined Patent Publication Hei. 8-258096, is constructed in general as follows. That is, the apparatus comprises a heating cylinder, a main screw provided inside the heating cylinder, a mixing screw provided on the top end portion of the main screw, and an inert gas supply unit for supplying an inert gas to the mixing screw portion. Thus, when the main screw is rotatably driven to feed pelletized resin material toward the top end portion of the heating cylinder, the pelletized resin material is melted and then further uniformly melted with the mixing screw. At this time, when a carbon dioxide gas is supplied to the melted resin, the carbon dioxide gas permeates the melted resin material. Injecting the melted resin material permeated by the carbon dioxide gas into a mold by driving the main screw in the axial direction will provide a fine foam. On the other hand, the apparatus for forming a thermoplastic resin foam, disclosed in Japanese Unexamined Patent Publication Hei. 10-230528, is constructed as follows. That is, the apparatus comprises two separate units such as a successive plasticizing unit having a heating cylinder and a screw, and an injecting unit having a plunger. Thus, the two units can also provide a thermoplastic resin foam as follows. That is, when the screw is rotatably driven to melt the pelletized resin material and a carbon dioxide gas is supplied to the melted resin, the carbon dioxide gas permeates the melted resin material. The melted resin material permeated by the carbon dioxide gas is injected into the mold of the injecting unit having the plunger by driving the screw in the axial direction. Then, the plunger is driven to provide a thermoplastic resin foam in the same manner.
However, for example, the critical pressure of the carbon dioxide gas is 7.4 MPa. There is an unsolved problem of sealing the carbon dioxide gas inside the heating cylinder, which will arise when the carbon dioxide gas under a supercritical state with a pressure considerably higher than the aforementioned pressure is injected into the melted resin material inside the heating cylinder. Furthermore, the apparatus for forming a thermoplastic resin foam is complicated in structure and comparatively expensive.